Project Summary/Abstract Postoperative cognitive decline (POCD) is a debilitating condition that particularly plagues aged individuals (~40% of elderly surgical patients develop POCD). POCD substantially increases the risk of morbidity, mortality, and the development of Alzheimer?s disease. Inflammation in the brain (?neuroinflammation?), which becomes more excitable or ?primed? with age, may cause POCD. Indeed, substantial changes in the immune system occur with age and the immune system communicates with the central nervous system (CNS) and alters the function of local CNS immune cells such as microglia. In modern society, unusually clean conditions decrease exposure to environmental and commensal microorganisms that help shape immunoregulatory circuits. Reintroduction of microorganisms in an overly sterile environment can improve immunoregulation and quell hyperactive inflammatory responses. Despite the therapeutic potential of specific small microbes, few studies have assessed whether microbial-based treatment strategies target the CNS and are effective in aged populations. One such microbe that could benefit neuroinflammation in aging is the widely distributed soil bacterium Mycobacterium vaccae (M. vaccae), which is approved for therapeutic use in humans and improves quality of life in other contexts (allergy and cancer). Thus, the central hypothesis of this proposal is that M. vaccae immunotherapy (three subcutaneous injections) will protect against primed neuroinflammation and cognitive decline in aged rats following surgery. Our preliminary results indicate that M. vaccae immunotherapy has robust and sustained anti- inflammatory activity (+IL-4) in the CNS of aged rats and blocks persistent pro-inflammatory responses (IL-1?) and cognitive deficits following surgery. M. vaccae appears to induce a beneficial (anti-inflammatory) CNS response and redirect the activation state of microglia. M. vaccae enhances immunoregulation in the body through altering T cell populations and may shift the population of T cells accessing the CNS and surrounding meningeal space to elicit changes in the CNS. Thus, this proposal addresses the following specific aims: first, establish whether M. vaccae is a viable pre-clinical target for preventing age-associated POCD; second, identify the CNS-specific mechanisms by which M. vaccae induces a sustained protective shift in the aged neuroimmune environment; and third, establish how M. vaccae communicates its anti-inflammatory signal to the brain. This research is innovative: microbial-based treatments can effectively improve peripheral inflammation, yet there is almost no research investigating whether microbial-based treatments can protect against damaging neuroinflammation. Our overall long-term goals are to elucidate the cellular and molecular mechanisms underlying neuroinflammatory priming; to understand how microglia function transitions toward pathological responses with age; and to determine how these neuroimmune alterations hijack behavioral function.